During many years, recovery of metal values from metallurgical waste materials has been developed. Such recovery is beneficial for many reasons. One is that waste materials, e.g. EAF (Electric Arc Furnace) dust, and slag of different kinds often contains such high amounts of heavy metals that they are unsuitable for immediate deposition. Rest products comprising elements like Sn, Zn and Pb are preferably not returned in to the nature without any protective treatment. Furthermore, the value of these elements is also not negligible. By recovering these elements, the environment is saved at the same time as useful metals are obtained.
It is since long known to use different types of fuming processes to recover evaporable substances both from primary sources, such as roasted Zn ores and from secondary sources, such as EAF dust, leaching residues and secondary slags. A typical simple slag fuming process produces a molten metallurgical slag. The slag is typically exposed to reducing agents and is heated to relative high temperatures. Vapours of volatile metals, such as e.g. zinc and lead, are transferred into the gas phase above the slag and the vapours are removed for further treatment to obtain the metallic components. One typical such example is found e.g. in the published patent application GB 2 181 746 A or the U.S. Pat. No. 5,942,023.
In the U.S. Pat. No. 4,571,260, a method for recovering metal values from materials containing tin and/or zinc is disclosed. The method is basically a Kaldo process where a surface of a rotating slag bath is exposed for oxygen and fuel. Flux and coke are added to achieve a suitable viscosity and appropriate agitation.
A disadvantage with most early slag fuming arrangements is that the efficiency in removing the volatile metals was not always the best. Relatively high contents of hazardous substances were remaining in the final slag.
Today literature and operational practice often mention and apply high temperature treatment of volatile containing materials. The high temperatures are necessary to ensure high fuming rates and high yields. As an example Zn fuming from fayalite slags can be used. Here the slag is superheated above its normal melting point of 1100° C. This superheating of the slag results in excellent fuming, however, it results also in high refractory wear and higher energy consumption of the process.
Water cooled vessels are typically used to overcome the short lifecycle of the refractory, however, at a high cost of heat loss that comes with it. Therefore the smelters typically have to compromise between high wear, low fuming rates and high heat losses.
The above mentioned issues with high refractory wear and high energy cost were addressed in published international patent application WO 2005/031014. There, high Zn fuming rates without any need to superheat the slag are described. According to that approach, the melting point of the slag is increased to 1300° C. by adding suitable fluxes. By doing so, there is limited or no need for the slag to be superheated to obtain high fuming rates. The reason is that the desired temperature for high fuming rates is typically around 1300° C., and since the melting point of the slag is around 1300° C., superheating is typically not necessary. Such a slag is said to build a protective freeze lining on top of the refractory on water cooled walls and this approach therefore minimizing the wear of the lining. The fuming process according to WO 2005/031014 however comes at the cost of energy needed to heat all of the slag volume in the reactor up to 1300° C., and at the cost for fluxes that need to be added to increase the melting point of the slag.
Another approach is disclosed in the published U.S. Pat. No. 4,252,563. There it is described a continuous of slag fuming process where the slag is fumed in two consecutive slag treatment zones. In the first zone, the slag is subjected to heat treatment for fuming off volatile, preferably sulphide bound, constituents. In the subsequent second furnace zone, the slag is subjected to reduction treatment, where oxides are reduced to their elementary form and fumed off. If the slag is subjected to further separation after the fuming process, it can be subjected to a further 3rd zone for copper recovery. However this 3rd zone has again to be heated since the temperature of the slag after zone 2 drops considerably. The slag temperature is adjusted in the first zone so that the reduction and fuming treatment in the second zone can be carried out essentially without any further heating of the slag. Preheated air and pulverized coal is used for heating the slag, giving the process a need for a robust off-gas system. Utilization of coal as fuel gives the process limitations for energy input and oxygen potential. At relatively strong reduction conditions, huge amounts of coal have to be supplied to cover the energy demand for heating and reduction. This gives rise to very high amounts of exhausted greenhouse gases.
The use of submerged plasma torches generating a gas agitating the slag bath and for feeding of reducing agents is known, e.g. as disclosed in the published US patent application US 2010/0050814.
Despite the development in this technical area, there are still remaining problems. In particular, by solving the earlier problems with lining wear and fuming rate, the costs for fluxes, heating and cooling is increased, as well as, in some cases, the high emissions of carbon oxides.